1. Field of the Invention
The present invention relates to a ceramic catalyst body capable of purifying an exhaust gas discharged from an internal combustion engine such as an automobile engine of an automobile.
2. Description of the Related Art
There is a ceramic catalyst body composed of a ceramic carrier which supports a catalyst therein capable of purifying an exhaust gas discharged from an internal combustion engine for a vehicle such as a diesel engine. Such a ceramic catalyst body has a plurality of cells therein. Each cell is partitioned or surrounded by porous cell walls which support catalyst particles thereon. A plurality of those cells in the ceramic catalyst body is arranged in a honeycomb shape.
The ceramic catalyst body is placed in the passage of an exhaust gas discharge pipe through which the exhaust gas is discharged from an internal combustion engine to the outside of a vehicle. The exhaust gas of a high temperature passing through the ceramic catalyst body enables the catalyst in the ceramic carrier to be activated.
However, such a ceramic catalyst body involves a following drawback. In a strict enforcement of a recent legal control in the field of vehicle exhaust gas, there has been proposed that the ceramic catalyst body is placed more closer to the engine in order to activate catalyst components supported in the ceramic catalyst body as fast as possible. This proposal intends to increase the temperature of the catalyst by the exhaust gas so that the catalyst components are activated more quickly.
Although increasing the temperature of the catalysts can promote the activation thereof, the catalyst components are easily separated from the ceramic carrier in the ceramic catalyst body under a high temperature condition. The separation of the catalyst components from the ceramic carrier causes deterioration in the durability of the ceramic catalyst body. For this reason, there is a strong requirement of providing a ceramic catalyst body capable of suppressing the separation of catalyst components from the ceramic carrier and capable of maintaining the purifying performance of the catalyst capable of purifying an exhaust gas even if used under a high temperature environment.
For example, Japanese patent laid open publication No. JP S64-4249 has disclosed a cordierite honeycomb-structure ceramic carrier having an improved catalyst-supporting capability which is achieved by adjusting various parameters of a honeycomb structure body, which is a base body of the cordierite honeycomb structure ceramic carrier, such as a porosity or a pore ratio, a pore distribution, and a thermal expansion coefficient.
Japanese patent laid open publication No. JP H9-262484 has disclosed a ceramic honeycomb catalyst having an improved anti-separation capability of catalyst by adjusting a thermal expansion coefficient of a ceramic honeycomb-structure base body and by adjusting a difference of a thermal expansion coefficient before and after the ceramic honeycomb-structure base body supports the catalyst.
Japanese patent laid open publication No. JP 2002-59009 has disclosed a cordierite honeycomb structure body having an improved adhesion capability between a catalyst and a coating layer made of γ-alumina and the like formed on a diffusion prevention layer by controlling a thickness of the diffusion prevention layer of being not more than an average pore size of the cordierite honeycomb structure body, where the diffusion prevention layer is formed on the surface of the cordierite honeycomb structure body and capable of preventing the diffusion of the catalyst and the diffusion prevention layer is formed on a surface of the cordierite honeycomb structure body and capable of preventing the diffusion of catalyst components.
Japanese patent laid open publication No. JP H6-165939 has disclosed a cordierite honeycomb structure body having an improved anti-thermal shock after it supports a high specific surface material, where improved anti-thermal shock is obtained by setting a thermal expansion coefficient of the cordierite honeycomb structure body to a value of not less than 1.0×10−6/° C. at a temperature range of 40 to 800° C.
Japanese patent laid open publication No. JP 2003-205246 has disclosed a method of producing a catalyst body of an improved strength. In the method, a ceramic carrier of a high porosity is firstly produced. The ceramic carrier is compacted by an after-treatment such as a coating step in order to obtain the ceramic carrier of a low porosity while suppressing the invasion of alkali metal.
Japanese patent laid open publication No. JP 2000-296340 has disclosed a ceramic honeycomb structure body capable of suppressing deterioration in its anti-thermal shock by controlling its characteristic as follows: The ratio of area of a pore within a range of not less than one time to not more than ⅓ times of an average particle size of a high specific area material is within a range of 1 to 12% when a minimum diameter of the pore on a surface of a cell wall observed from a vertical direction to the surface; and the ratio of area of a pore having a minimum radius of not less than 10% of an average particle size of a high specific area material is not more than 10% when the pore on the surface of the cell wall is observed in a direction which is vertical to the surface of the cell wall.
Japanese patent laid open publication No. JP 2002-191985 has disclosed a cordierite honeycomb structure body having an improved adhesion of a highly specific-area material by controlling an amount of pores having a diameter of 0.5 to 5 μm to become not more than 30% of a total amount of pores.
However, because those techniques disclosed in the above related-art documents do not always suppress the separation of catalyst components from a ceramic carrier, there is a strong demand to provide a ceramic catalyst body of superior durability and reliability capable of adequately maintaining the anti-separation performance from the ceramic carrier.